There are three inter-related questions about a discussion of free will which are central to any reasoning upon the subject. First there is the question of what "free will" actually means. Second, there is the question of whether we actually have a free will. Third, there is the question of whether or not having a free will is compatible or incompatible with the thesis of determinism. The answer that philosophers give to the first question will directly influence how they address the second and third.

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Incompatibilism means that the notion of a deterministic universe is completely at odds with the notion that people have a free will. It can be treated in at least two ways: by libertarians, who deny that the universe is deterministic through-and-through, and the hard determinists, who deny that any free will exists.

Libertarianism suggests that we actually do have free will, and that therefore the future is not determined. That is, at this moment, I am correct in thinking that I could either delete this article if I wanted, or continue editing it. Since it is currently the case that I could do either, the fact of how the history of the world will continue to unfold is not currently determined one way or the other. One famous proponent of this view was Lucretius, who asserted that the free will arises out of the random, chaotic movements of atoms, called "clinamen".

The major objection to this view is that science (before quantum mechanics) has gradually shown that more and more of the physical world obeys completely deterministic laws, and seems to suggest that our minds are just as much part of the physical world as anything else. Some libertarians (perhaps like Roger Penrose) may suggest that some quantum indeterminism in the brain gives rise to this freedom, even though most of the rest of the large-scale universe is apparently deterministic. Others may use some form of Donald Davidson's anomalous monism to suggest that although the mind is in fact part of the physical world, it involves a different level of description of the same facts, so that although there are deterministic laws under the physical description, there are no such laws under the mental description, and thus our actions are free and not determined.

But there are obvious tensions in such positions, and it seems a bit stubborn to resist scientifically-motivated determinism on purely intuitive grounds about our own freedom, no matter how obvious it seems to some.

Some are willing to go along with the history of the development of science and suggest that determinism is true, and because free will seems to imply that the future is not determined (because I could freely act one way or another), this means that free will doesn't actually exist. Instead, it is just an illusion furthered by our ignorance. Since many believe that free will is necessary for moral responsibility, this would have disastrous consequences for their theory of ethics.

Some people go farther and suggest that even non-determinism might be incompatible with free will. This is pessimistic incompatibilism. The idea is that if the universe is determined, then there is no free will, as mentioned above. However, if it is not determined (say, for quantum mechanical reasons), then it seems reasonable to suggest that any event has a probability assigned to it. But if there is a probability that I will delete this article (say 1%), then whether or not I delete it seems not to be a free choice on my part, but rather a brute random fact about the world. On this account, the notion of free will seems to be just a conceptual confusion we have, that couldn't possibly exist, whether the universe was deterministic or not.

Compatibilism, most famously championed by Hume, is a theory that suggests that free will and determinism are in fact compatible. According to Hume, free will should not be understood as an absolute ability to have chosen differently under exactly the same inner and outer circumstances. Rather, it is a hypothetical ability to have chosen differently if one had been differently psychologically disposed by some different beliefs or desires. That is, when I say that I could either continue to edit this page or to delete it, I don't really mean that both choices are compatible with the complete state of the world right now, but rather that if I had desired to delete it I would have, even though as a matter of fact I actually desire to continue editing it, and therefore that is what will actually happen.

Hume also maintains that free acts are not uncaused (or mysteriously self-caused as Kant would have it) but rather caused by our choices as determined by our beliefs, desires, and by our characters. While a decision making process exists in Hume's determinism, this process is governed by a causal chain of events. For example, a person may make the decision to support Wikipedia, but that decision is determined by the conditions that existed prior to the decision being made.

One libertarian or incompatibilist reaction to this claim holds that "free will" refers to genuine (e.g. absolute, ultimate) alternate possibilities for beliefs, desires or actions, rather than merely counterfactual ones. In the absence of such possibilities, the belief that free will confers responsibility is held to be false. However, a compatibilist may respond with the argument mentioned above stating that non-determinism is also incompatible with free will, so the libertarian is no better off. They may also argue on conceptual grounds that "free will" has nothing to do with ultimate causes on a grand metaphysical scale, but instead only refers to an apparent fact of human psychology (i.e., that conscious mental states seem to play an active role in determining the choices that are made).

Compatibilists often continue and argue that determinism is not just compatible with free will, but actually necessary for it. If my actions aren't determined by my beliefs, my desires, and my character, then it seems that they aren't really my actions.

William James, the Americanpragmatist philosopher who coined the term "soft determinist" in an influential essay titled The Dilemma of Determinism, held that the importance of the issue of determinism is not one of personal responsibility, but one of hope. He believed that thorough-going determinism leads either to a bleak pessimism or to a degenerate subjectivism in moral judgment. The way to escape that dilemma is to allow a role of chance. He said that he would not insist upon the name "free will" as a synonym for the role chance plays in human actions, simply because he preferred to debate about objects, not words.

An argument can be made which claims that the aspects of reality that are important to hope are unaffected by determinism. Whether or not the universe is determined does not change the fact that the future is unknown, and that a person's actions help determine that future. In fact, it is even conceivable that a lack of belief in determinism could lead to 'bleak pessimism', since one could potentially believe that their actions did nothing to determine future events.